Aircraft, during flight and/or while on the ground, may encounter atmospheric conditions that cause the formation of ice on airfoils and other surfaces of the aircraft structure, including wings, stabilizers, rudder, ailerons, engine inlets, propellers, rotors, fuselage and the like. Accumulating ice, if not removed, can add excessive weight to the aircraft and alter the airfoil configuration, causing undesirable and/or dangerous flying conditions. Since the earliest days of flight, attempts have been made to overcome the problem of ice accumulation. However, available de-ice and/or anti-ice systems have usually been reserved for large aircraft because expense and added weight have made them impractical for general aviation single engine and light twin airplanes and helicopters.
Several approaches have been used to provide de-ice and/or anti-ice systems for modern aircraft and these are generally categorized as mechanical, chemical or thermal. One commonly used system for de-icing is the mechanical removal of ice with pneumatic boots. In this system, the leading edge zone or wing or strut component of an aircraft is covered with a plurality of expandable cuffs that are inflatable with a pressurized fluid, such as air. When inflated, the cuffs expand and crack accumulating ice which is then dispersed into the air stream. Although pneumatic boots have been used on commercial aircraft and some light twin or small jet aircraft, the system, which requires an air compressor and vacuum system, is expensive and adds appreciable weight to light aircraft. A similar mechanical system employs multiple juxtaposed electro-expulsive elements placed within an elastomeric or metal clad boot that is attachable to airfoil surfaces. When an electrical impulse is applied, the force effects an impulse separation of one element from the other that is sufficient to mechanically throw off thin accretions of ice. In each of these mechanical systems, boot operation affects the airfoil characteristics of the wing, with the result that the boots cannot be operated during landing or takeoff.
Another mechanical system for removing ice that has already formed employs a composite leading edge with a titanium skin. Under the skin are located tubes through which air is pulsed at high force, creating a shock wave that throws off thin accretions of ice into the air stream. Although this system is lighter than either the pneumatic boot or the electro-expulsive system, it is also expensive for small aircraft because of the expense of the titanium skin.
In addition to the drawbacks of the added weight and expense of current mechanical ice removal systems, each of these systems requires visual attention to the degree of ice build-up and careful timing of activation for maximum effectiveness. Moreover, none of these systems are suitable for use as an anti-ice system (i.e. to prevent ice from forming).
Another common approach for de-icing and/or anti-icing aircraft surfaces involves the application of a chemical, such as alcohol, to reduce adhesion forces associated with ice accumulation and/or depress the freezing point of water collecting on the surfaces. Such systems may be used while the aircraft is on the ground or in flight. For example, one system prevents the build-up of ice on the leading edges of airfoil surfaces, including propeller and rotor blades, by the weeping of alcohol from a plurality of holes in an attached titanium cuff. Drawbacks of such on-board chemical systems include their expense and the necessity to rely on a finite supply of chemical during flight.
Potential thermal anti-ice or de-ice systems have been reported. One such system, limited to turbine aircraft, diverts bleed air or hot air from one of the turbine stages to heat the airfoil leading edges. Other thermal systems employ electrically conducting resistance heating elements, such as those contained in heating pads bonded to the leading edges of the aircraft or on the propeller or rotor blades, or those incorporated into the structural members of the aircraft. Heating pads of this type usually consist of an electrically conductive material in contact with wire or other metal heating elements dispersed throughout the conductive layer which is sandwiched between two layers of insulation. Electrical energy for the heating elements is derived from a generating source driven by one or more of the aircraft engines. The electrical energy is continuously supplied to provide heat sufficient to prevent the formation of ice or intermittently supplied to loosen accumulating ice. However, such systems are only usable where sufficient wattage is available to raise and/or maintain the temperature of the airfoil surface above the freezing point at typical aircraft speeds. For example, an anti-ice system that is continuously on during icing conditions should ideally maintain a surface temperature of approximately 100.degree. F. to 180.degree. F., as suggested by The National Aeronautic and Space Administration (NASA), or, more typically, at 100.degree. F. to 130.degree. F., as required for the Gulfstream IV aircraft, stated in the Gulfstream Pilot Operating Handbook. An ideal thermal de-ice system should be capable of maintaining a temperature of 100.degree. F. to 150.degree. F. during icing conditions.
To achieve temperatures such as those described above, the power supply required to power a wire heating pad type anti-ice or de-ice system is large, because of the inefficiencies of the resistive heating pad elements. For example, the wattage required for an anti-ice system in a typical high-performance single engine or light twin aircraft, using the above-described resistance heaters, is approximately 21,000 watts. Current power systems in such aircraft can supply a maximum of only about 7,000 watts. A typical general aviation light aircraft with an approximate anti-ice area of 1,400 square inches, using pad heaters, requires approximately 15 watts per square inch to reach anti-ice temperatures. Known heating pad systems supply only 2 to 3 watts per square inch at the power supplied by these aircraft. Therefore, typical resistance pad heaters have not been a feasible alternative as anti-ice or de-ice systems for general aviation aircraft.
Another drawback to typical thermal systems is the tendency for the protective covering to break down due to cutting, abrasion and erosion, causing heating wires to break. One thermal system attempts to overcome this problem by employing an epoxy composite material containing an integral heating layer comprising a mat of non-woven conductive metal-coated fibers, such as nickel-coated chopped graphite fibers, whose random orientation ensures that electrical connection is maintained if a single, or even many, connections are broken. However, this system requires extensive preparation of aluminum aircraft surfaces before application.
In view of the foregoing, there is a need for an inexpensive and efficient on-board system that provides both de-ice and anti-ice capabilities for general aviation aircraft, that is light in weight, that operates efficiently using the power supply currently available on these aircraft, that does not change the contour of the airfoil surface, and that is abrasion and wear resistant.